1. Field of the Invention
The present invention relates generally to split-cycle internal combustion engines also known as split-cycle engines and, more specifically, to a Double Piston Cycle Engine (DPCE) that is more efficient than conventional combustion engines.
2. Description of the Related Art
Conventional internal combustion engines include one or more cylinders. Each cylinder includes a single piston that performs four strokes, commonly referred to as the intake, compression, combustion/power/expansion, and exhaust strokes. Together, these four strokes form a complete cycle of a conventional internal combustion engine. However, a single cylinder cannot be optimized both as a compressor (requires cold environment for optimal efficiency performance) and a combustor (requires hot environment and optimal expansion of the working fluid for optimal efficiency performance) at the same time and space.
Conventional internal combustion engines have low fuel efficiency—more than one half of the potential thermal energy created by conventional engines is estimated to dissipate through the engine structure and exhaust outlet, without adding any useful mechanical work. A major cause of thermal waste in conventional internal combustion engines is the essential cooling system (e.g., radiator), which alone dissipates heat at a greater rate and quantity than the total heat actually transformed into useful work. Furthermore, conventional internal combustion engines are able to increase efficiencies only to a low degree by employing low heat rejection methods in the cylinders and pistons.
Further inefficiency results from high-temperature in the cylinder during the intake and compression strokes. This high temperature reduces engine volumetric efficiency, makes the piston work harder and, hence, reduces efficiency during these strokes.
Theoretically, a larger expansion ratio than compression ratio will greatly increase engine efficiency in an internal combustion engine. In conventional internal combustion engines, the expansion ratio is largely dependent on the compression ratio. Moreover, conventional means to make the engine expansion ratio larger than the compression ratio (Miller and Atkinson cycles, for example) are less efficient than the increase in efficiency, which is possible if all four strokes would have not been executed in a single cylinder.
Another problem with conventional internal combustion engines is an incomplete chemical combustion process, which reduces efficiency and causes harmful exhaust emissions.
To address these problems, others have previously disclosed dual-piston combustion engine configurations. For example, U.S. Pat. No. 1,372,216 to Casaday discloses a dual piston combustion engine in which cylinders and pistons are arranged in respective pairs. The piston of the firing cylinder moves in advance of the piston of the compression cylinder. U.S. Pat. No. 3,880,126 to Thurston et al. discloses a two-stroke cycle split-cylinder internal combustion engine. The piston of the induction cylinder moves somewhat less than one-half stroke in advance of the piston of the power cylinder. The induction cylinder compresses a charge, and transfers the charge to the power cylinder where it is mixed with a residual charge of burned products from the previous cycle, and further compressed before igniting. U.S. Pat. Application No. 2003/0015171 A1 to Scuderi discloses a four-stroke cycle internal combustion engine. A power piston within a first cylinder is connected to a crankshaft and performs power and exhaust strokes of the four-stroke cycle. A compression piston within a second cylinder is also connected to the crankshaft and performs the intake and compression strokes of the same four-stroke cycle during the same rotation of the crankshaft. The power piston of the first cylinder moves in advance of the compression piston of the second cylinder. U.S. Pat. No. 6,880,501 to Suh et al. discloses an internal combustion engine that has a pair of cylinders, each cylinder containing a piston connected to a crankshaft. One cylinder is adapted for intake and compression strokes. The other cylinder is adapted for power and exhaust strokes. U.S. Pat. No. 5,546,897 to Brackett discloses a multi-cylinder reciprocating piston internal combustion engine that can perform a two, four, or diesel engine power cycle.
However, these references fail to disclose how to differentiate cylinder temperatures to effectively isolate the firing (power) cylinders from the compression cylinders and from the surrounding environment. In addition, these references fail to disclose how to minimize mutual temperature influence between the cylinders and the surrounding environment. Further, these references fail to disclose engine improvements that enhance conventional internal combustion engine efficiency and performance by raising the power cylinder temperature and lowering the compression cylinder temperature. Specifically, increasing power cylinder temperature allows for increased kinetic work extraction, while minimizing compression cylinder temperature allows for reduced energy investment. In addition, the separate cylinders disclosed in these references are all connected by a transfer valve or intermediate passageway (connecting tube) of some sort that yields substantial volume of “dead space” between cylinders.
U.S. Pat. No. 5,623,894 to Clarke discloses a dual compression and dual expansion internal combustion engine. An internal housing, containing two pistons, moves within an external housing thus forming separate chambers for compression and expansion. However, Clarke contains a single chamber that executes all of the engine strokes. As noted above, a single chamber prevents isolation and/or improved temperature differentiation of cylinders such as those disclosed in embodiments of the present invention.
U.S. Pat. No. 3,959,974 to Thomas discloses an internal combustion engine including a combustion cylinder constructed, in part, of material capable of withstanding high temperatures and a power piston having a ringless section, also capable of withstanding high temperatures, connected to a ringed section, which maintains a relatively low temperature. However, elevated temperatures in the entire Thomas engine reside not only throughout the combustion and exhaust strokes, but also during part of the compression stroke.